Plant Autographs and Their Revelations

Returning to the question of consciousness, we commonly say that our favourite animals, dogs for instance, possess it. This we realise through our sympathy; but is a fish con¬ scious? Some say yes, and others no. In the animal king¬ dom, where does consciousness enter into life? Bergson dis¬ cusses this problem in his ‘Mind Energy’ : ‘It by no means follows that a brain is indispensable to consciousness. The lower we go in the animal series, the more the nervous centres are simplified and separate from one another, and at last they disappear altogether, merged in the general mass of an organism with hardly any differentiation. If, then, at the top of the scale of living beings, conscious¬ ness is attached to very complicated nervous centres, must we not suppose that it accompanies the nervous system down its whole descent, and that when the nerve stuff is merged in the yet undifferentiated living matter, consciousness is still there, diffused, but not reduced to nothing? Theoretically, then, everything living might be conscious. In principle conscious¬ ness is coextensive with life. The amoeba, when in presence of a substance which can be made food, pushes out towards it filaments able to seize and enfold foreign bodies. These pseudopodia are real organs and therefore mechanism; but they are only temporary organs created for the particular purpose. ... It appears extremely likely that conscious¬ ness, originally immanent in all that lives, is dormant where there is no longer spontaneous movement, and awakens when life tends to free activity.’

Consciousness in animal life is thus regarded as asso¬ ciated with spontaneous or voluntary movement due to internal hidden causes. It is also associated with nervous reactions. These two important characteristics have been regarded as totally absent in the plant. Leaving aside metaphysical questions on which authorities are so greatly divided, we shall deal with experimental facts and their implications. It will be shown that there is no characteristic of the higher animal which has not been fore¬ shadowed in the plant. We shall find that all plants, even rigid trees, perceive and visibly respond to stimuli from with¬ out; that even spontaneous movement, which is so charac¬ teristic of animal-life, is not absent, nor even dormant, but actively present in plant-life. I shall describe experiments in a later chapter proving that many plants possess a nervous system which shows a high degree of differentiation.

The plants, moreover, are possessed of remarkable sense- organs, by the guidance of which different parts of the body of the plant place themselves in favourable attitudes towards their environment, such adjustments reminding us of animal behaviour. In man himself, from the moment the ovum is fertilised until he becomes fully grown, when does consciousness come into existence? Do we not see that it must be there all the time, at first latent, yet waxing imperceptibly from this la¬ tency to full blossoming? In other words, there is then a continuous evolution from rudimentary beginnings to higher perfection. Hence there is no humiliation in our kinship with the lowest of the low; rather is it a matter of pride for man to have arisen through struggle in the scale of being from a mass of protoplasmic jelly to his present state.

The contraction of the sensitive pulvinus of Mimosa enables us to obtain a record of mechanical response of the plant. The functional similarity between the two contractile organs, pulvinus and muscle, is not confined to the manifestation of outward movement, but can be traced to the ultimate proto¬ plasmic mechanism. In regard to rapidity of movement in animal muscle, the tortoise and the hare present evident differences ; in the swift animal the reaction must be rapid, while in the slug¬ gish, the response is slow. The wing-muscle of a bird of prey like the falcon is very active; that of the goose is less active, while the muscle of the domestic fowl is almost inac¬ tive, its power of flight being practically lost. What is it that confers extreme rapidity of action? Curiously enough, in the leaves of plants also there are three types of motor organ — active, semi-active, and inactive — corresponding to the three types in the animal.

In Mimosa pudica the rate of movement in response to stimulation is very rapid, the contractile fall being completed in a time as short as a second. In other sensitive plants the rate of movement is comparatively slow, as is the case with Neptiinia oleracea, not very unlike Mimosa pudica in appear¬ ance. Neptunia flourishes in lakes, and in order to float in water it grows a cork belt round its stems. The movement of the leaf of Neptunia is so slow that the time taken for the leaf to complete its fall is more than a minute. Finally, the

Fig. 13. Longitudinal section of petiole and pulvinus of Mimosa passing through the upper and lower vascular bundles. Pt, the cortical cells of the petiole which remain unstained. Lower P to the right, contractile cells of pulvinus deeply stained. shows a movement which is very feeble and ex¬ tremely sluggish. Turning our attention to the motile organ, it is the contraction of the cor¬ tical cells of the pulvinus of Mimosa that produces the rapid fall of the leaf. The pulvinus of the Bean- plant is, as already stated, inactive, though its corti¬ cal cells are anatomically similar to those of Mi¬ mosa. Anatomical simi¬ larities have hitherto been unduly pressed in deter¬ mining the true function of an organ; the facts given above will show the misleading character of such a criterion. It is not anatomical structure, but the protoplasmic content of its tissue that confers on an organ its physio¬ logical efficiency for dis¬ charge of a specific func¬ tion. The motility of an organ is, as we shall pres¬ ently find, dependent on

the presence of a certain active substance in the proto¬ plasm. How is the presence of an active substance to be ascer¬ tained? If we examine a longitudinal section of the pul- vinus of Mimosa under the microscope, it is impossible to observe where the contractile cells begin, where they end, and how they are distributed. I succeeded, however, in dis¬ tinguishing them by selective staining with safranin. This stain produced the most remarkable results; it appeared as if a hand had, with utmost care, picked out every actively contractile cell and painted its protoplasmic contents a deep crimson (fig. 13).

The outline of the contractile tissue was now sharply defined; the stained protoplasmic contents of the active cells showed under high microscopic magnification a marked granular appearance. In the semi-active Neptunia, the stained contractile cells were found not to be compact as in Mimosa, but scattered in their distribution. No staining occurred in the pulvinar cells of the inactive Bean-plant, the active substance being here altogether absent (fig. 14). It is very remarkable that the activity of animal muscle is similarly dependent on the presence and relative distribution of an active substance (fig. 15). The pulvinus of Mimosa may thus be regarded as functionally equivalent to an active animal muscle.

All movements are ultimately dependent on oxidation or combustion. In an engine, the speed of the moving machin¬ ery is proportional to the rate of consumption of the fuel, the higher the rate of combustion, the greater the rapidity of movement. In the living machine also, the rate of oxi¬ dation plays an important part in the production of rapid movement, and in the present case the active substance is found to be highly oxidisable. We shall next make a further study of the variation of response in plants under changed external conditions. It was shown in the last chapter how animal-like, even human-

like, are the general reactions of plants, and how, like us, they are affected by light and darkness, warmth and cold, rest and fatigue. There are also other factors which pro- Fig. 14. Reproduction of photomicrographs of portions of the pulvini of (a) active Mimosa, (b) semi-active Neptunia, and (c) inactive Bean-plant. The stained content of the contractile cells repre¬ sented dark. Fig. 15. Transverse sections of pectoral muscles of (a) the falcon, (b) the goose, and (c) the domestic fowl. The relative amount of granular substance (shown as shaded) varies directly with the bird’s power of sustained flight (after Knoll).

foundly affect us, for example, the purity or impurity of the air we breathe. Again, there are various drugs which exert characteristic effects upon us, now beneficial, now detri¬ mental. Let us take, for instance, the air we breathe. The vitiated air of the town causes in us depression and lowers the gen- eral tone, whereas a trip to the country, with its ozone-laden air under the pine-trees, restores health and vigour. Think of the harmfulness of a closed stuffy room and the necessity for good ventilation. Think also of the evil habits of man, as he takes to alcohol and alcohol takes him.

Then there are the various narcotics which we take to induce sleep or to make us unconscious of the surgeon’s knife. Some of these drugs, ether, for example, are prac¬ tically safe ; for a man may be rendered unconscious and be afterwards revived by blowing off the vapour. Chloroform is a more powerful narcotic, and though it is highly efficient, the dose may easily be exceeded with even fatal results. Now let us enquire whether all or any of these reactions, so specific in human and animal life, have their correspon¬ dence in plant-life. We have first to find a means by which plants can be subjected to the action of different gases and narcotic vapours. For this, we enclose the plant in a glass chamber with an inlet and an outlet pipe. Gases and vapours can then be pumped into the chamber, air being expelled through the exit pipe. In order to observe the after-effect, the chamber can then be filled with fresh air. If the gases have produced only temporary narcotisation of the plant, then the introduction of fresh air revives it. But if the dose has exceeded the safety limit, or if the gas has been too poisonous, death supervenes without possibility of revival.

In the case of liquid drugs, we apply them at the roots, or at the cut end of the stem, and the suction exerted bv the plant causes them to spread throughout the tissues. I shall first describe the effect of carbonic acid gas. Accord¬ ing to popular belief, what is death to the animal is supposed to be life for the plant. The plant is assumed to flourish in the deadly atmosphere of carbonic acid gas. The record, however, shows that, instead of flourishing, the plant is suffocated just like a human being. Note the gasp of relief when fresh air is introduced! (fig. 16). We must remem-

ber, in this connection, that most living things, including plants, require oxygen for respiration. Unlike animals, however, green plants, in the presence of light, are able to decompose the carbonic acid gas which they take in from the air, giving off its oxygen and fixing its carbon for their nutrition. This power of carbon-assimilation is in no wise to be confused with the taking in of oxygen for respiration. The latter is an indispensable condition for the maintenance of both plant and animal life.

In contrast to the effect of carbonic acid gas is the invig¬ orating effect of ozone, which causes an enhancement of the response. It is well known that many plants do not flourish in the atmosphere of a town. For example, the sensitive plant Biophyfum sensitivum, in which the minute leaflets are thrown into a state of flutter by the least excitation, could not be kept in a lively condition in my laboratory in Cal¬ cutta. I found it to be fully alive and in a highly sensitive

condition in the suburbs at a distance of seven miles from the city. The city air contains traces of various gases, such as sulphur dioxide and sulphuretted hydrogen, both of which are highly injurious to the plant. This is seen in fig. 17. Here the introduction of sulphuretted hydrogen abolished all sensibility ; the three thick dots repre¬ sent the application of strong shocks, to which there was no response. From such deadly poisons we turn to narcot¬ ics. First we try ether. The record (fig, 18) shows that after an application the plant begins to lose its excitability, just as a person loses consciousness after etherisation. On

blowing off the narcotic vapour, the plant is seen gradually to regain its normal sensibility. The results of other experiments show that a very minute dose of ether causes an increase of excitability. Here we come across the remarkable fact that the effect of a drug is changed by the amount of the dose applied; a minute dose, generally speaking, produces a result diametrically opposite to that of a larger dose. than cholorform, any excess of which is attended by fatal results. In fig. 19 is shown the effect of a large dose of choloroform on the plant. This not only produced a total abolition of excitability, but brought about a sudden spasm which is seen as a line shooting upwards. After this, the blowing off of the vapour failed to revive the plant, which showed the characteristic discoloration of death.

The immediate effect of dilute vapour of alcohol is often to produce a transient enhancement of excitability. But a depression results from its continued action. The ludi¬ crously unsteady gait of the plant under intoxication (fig. 20) could, no doubt, be effectively exploited in a tem¬ perance lecture! A LIVING tissue is said to be sensitive because it responds to stimulation from outside. As long as it is alive, so long will it respond and then recover, making itself once more ready for a new response. The brief disturbance of the living poise, to be restored to equilibrium of itself, is quite unlike the rolling of a stone downhill after a push. For the stone cannot of itself regain its original position; but the living tissue quickly reasserts its stable poise on the ces¬ sation of the stimulation.

The response of the plant to stimulation, as recorded in the foregoing chapters, has been that of mechanical move¬ ment effected by the contraction of a motile organ. The intensity of the response was found to depend on the pres¬ ence of a certain active substance in the cells of the organ. Vegetable tissues in general are, however, physically restrained from exhibiting that responsive movement which we have hitherto accepted as a test of the sensibility of a tissue. The very important question arises as to whether a tissue which cannot show any movement is nevertheless sensitive to stimulation ; whether all plants and their different organs, leaf stem, root, flower and fruit, are irritable. In order to answer this question, it is necessary to devise a method by which excitation in living organs may be detected even in the absence of movement.

Movement is not the only manifestation of excitatory reaction. Stimulation induces an electrical change in an irritable tissue. If we take a piece of living stem, any two points on its surface, A and B, will be found to be in the same electrical condition, provided both are in a state of rest. The similarity of electrical condition at the two points A and B may be demonstrated by suitably connecting them with a galvanometer, which is a sensitive detector of elec¬ trical current. When the two points are at rest, they are in the same electrical condition, and therefore no current flows through the galvanometer. If next we excite the point B, say by pinching or striking it, then the electrical level of B in relation to A will be disturbed, and an electrical cur¬ rent will be found to flow from A to B through the (a)

galvanometer, which will show a deflection. The excited point B will be galvanometrically nega¬ tive with respect to A. Stimulation of A gives rise to a current in the opposite direction (fig. 21 ). For the sake of simplicity, we shall hence¬ forth describe the electri¬ cal change of the excited point as negative. After a time, as the tissue re¬ covers, the electric change, due to excitation, will also disappear. The following method was adopted to record the induced electrical changes. The galvanometer which is to record the responsive electric current consists of a coil of wire in the centre of which is suspended a small magnet with its length parallel to the plane of the coil. When a current flows round the coil, the magnet rotates in one direction or another, depending on the direction of the current. A small mirror is attached to the suspended magnet, and a spot of light reflected from the mirror magnifies the rotation of the magnet. The reflected ray of light acts as a long magni¬ fying index without weight. For inscribing the record we

(b) Stimulation at A produces a current of response from B to A across the galvanometer (up- curve) ; stimulation at B gives rise to a current in reverse direction (down-curve). use a sensitive plate, and the moving ray of light photo¬ graphically records the response and recovery. We shall presently find that the electrical pulse of response is a faith¬ ful indicator of the vitality of a tissue, the electrical response disappearing at the death of the tissue.

Before proceeding further, it is desirable to get a clear conception of the various manifestations of excitation or its opposite. Under normal conditions, the tissue of the plant is tense or turgid with sap, it is in a state of turgor. After stimulation various symptoms appear as so many signs of excitation. These are (i) contraction, (2) dimi¬ nution of turgor, (3) movement such as the fall of the leaf in Mimosa, and (4) negative electric change. The opposite process, which brings about recovery, and may even carry it to excess of recovery, is characterised by ( i ) expansion, (2) increase of turgor, (3) movement of erection as in leaf of Mimosa, and (4) positive electrical change.

Having explained the general principles, we shall next enter upon the practical method of obtaining electrical response. The most difficult problem in stimulating the plant is to keep the stimulus constant, or to increase or decrease it in a graduated manner. The most satisfactory method of stimulation, that of an electric shock, could not be utilised in the present case, because the leakage of the shock-current would disturb the current of response. Hence a non-electrical method of stimulation by torsional vibra¬ tion had to be devised.

If we hold our finger and cause a slow twisting move¬ ment, the stimulation produced will be slight; but if the torsion be made very suddenly, the stimulus will be intense, and cause a painful sensation. The efficiency of a stimulus is, in general, found to depend on the abruptness of its impact. In experimenting with plants, we take a piece of stem, A B, and hold it in the middle by a vice, C, which acts as a block, preventing excitation of one-half from passing

over into the other. One of the ends, say B, is held in three clamping jaws H. A torsional vibration is now imparted to this end by the handle K. The angle of torsional vibra¬ tion, which determines the intensity of stimulus, can be accurately measured by the graduated circle and may be predetermined by a sliding stop. Electrical connections are made with A and B through E, E , which lead to the gal¬ vanometer. The plant is placed in a glass chamber in which

Fig. 22. Apparatus for observing modification of electric response it can be subjected to the action of high temperature by means of heating the electric coil R. For experiments on the action of anaesthetics, the vapour can be blown through the side tube (fig. 22). I shall now give records of the electric response of the plant and its modification under varied external conditions. As regards sensitiveness in ordinary plants, we cannot imagine anything more stolid and undemonstrative than a carrot. It is a revelation to find how excitable it is, and how vigorous and uniform are its successive responses even through a long period (fig. 23).

Fig. 23. Uniform electric responses under torsional stimulus (Carrot). The Carrot has been shown to give a long series of uni¬ form responses. Certain plants, on the other hand, are easily fatigued. Celery, for instance. But all plants show fatigue when the intervening periods of rest are shortened (fig. 24). A similar effect was found in the mechanical Fig. 24. Fatigue of electric response under shortened period of rest. Turning to the minimum temperature which is fatal, tropical plants succumb to it earlier. Thus the tropi¬ cal Eucharis lily, when sub¬ jected to a freezing tem¬ perature for fifteen min¬ utes, had its electrical response completely abol-

ished, whereas northern plants like Holly and Ivy, when subjected to the same temperature, continued to give their electric response. I took twenty leaf-stalks of Horse-chestnut and divided them into two batches of ten each. One batch was kept in water to serve as control, and the second batch had the cut ends placed in solution of mercuric chloride, a well-known poison. The plants had numerous plant-lice living on them. It was observed after a period of twenty-four hours that the average electric response of the leaf-stalks standing in water was twenty-three divisions of the galvanometric scale, and that the lice on them were still living. On the other hand the leaf -stalks that had been standing in solu¬ tion of mercuric chloride gave no electric response on stimu¬ lation. Evidently they had been killed by the poison, a con-

elusion strengthened by the fact that the lice upon them were dead. In the cases given above, it was only the last fatal phase that was observed. It appeared important, however, to trace tne gradual effect of the administration of a poison culminating in death. The method adopted was to obtain first a series of normal responses to uniform stimulation; subsequently, without interrupting the procedure, the poi¬ sonous agent, chloroform vapour, was blown into the plant- chamber.